Method and apparatus for fast automatic centerline extraction for virtual endoscopy

ABSTRACT

A method for automatic centerline extraction for a virtual endoscopy image of an organ having a boundary surface includes centering on selected points of an initial path through the image, which is derived from an endoscopy dataset, respective spheres exhibiting respective maximal diameters short of contacting the boundary surface; and forming a centered path consecutively joining centers of the spheres.

CROSS-REFERENCE TO RELATED APPLICATIONS PRIORITY

Specific reference is hereby made to U.S. Provisional Application No.60/470,579, (Attorney Docket No. 2003P06954US), entitled FAST AUTOMATICCENTERLINE EXTRACTION, filed May 14, 2003 in the name of Bernhard Geigeret al., the inventors in the present application and of which thebenefit of priority is claimed and whereof the disclosure is herebyincorporated herein by reference.

Reference is also made to copending U.S. patent application Ser. No.10/753,703, entitled METHOD AND APPARATUS FOR AUTOMATIC LOCAL PATHPLANNING FOR VIRTUAL COLONOSCOPY, filed Jan. 8, 2004 in the name ofBernhard Geiger, an inventor in the present application and whereof thedisclosure is hereby incorporated herein by reference to the extent itis not incompatible with the present invention.

The present application relates generally to computer vision and imagingsystems, virtual endoscopy and, more particularly, to a system andmethod for fast, automatic centerline extraction such as may be utilizedfor virtual endoscopy, including virtual colonoscopy.

BACKGROUND OF THE INVENTION

Virtual colonoscopy (VC) refers to a method of diagnosis based oncomputer simulation of standard, minimally invasive endoscopicprocedures using patient specific three-dimensional (3D) anatomic datasets. Examples of current endoscopic procedures include bronchoscopy,sinusoscopy, upper gastro-intestinal endoscopy, colonoscopy, cystoscopy,cardioscopy, and urethroscopy. VC visualization of non-invasivelyobtained patient specific anatomic structures avoids risks, such asperforation, infection, hemorrhage, and so forth, associated with realendoscopy, and provides the endoscopist with important information priorto performing an actual endoscopic examination. Such information andunderstanding can minimize procedural difficulties, decrease patientmorbidity, enhance training and foster a better understanding oftherapeutic results.

In virtual endoscopy, 3D images are created from two-dimensional (2D)computerized tomography (CT) or magnetic resonance (MR) data, forexample, by volume rendering. Present-day CT and MRI scanners typicallyproduce a set of cross-sectional images which, in combination, produce aset of volume data. These 3D images are created to simulate imagescoming from an actual endoscope, such as a fiber optic endoscope.

It is desirable in virtual endoscopy, and especially in virtualcolonoscopy, to determine a centerline as guide to the examinationprocedure. Prior techniques for calculating such a centerline typicallyutilize concepts of morphological operators, distance transform, minimumcost path, Dijkstra's algorithm, and so forth. References to such priortechniques can be found, for example, in Zhou et al., “Three-DimensionalSkeleton and Centerline Generation Based on an Approximate MinimumDistance Field,” The Visual Computer, 14:303-314 (1998); Truyen, T.Deschamps, L. D. Cohen. Clinical evaluation of an automatic path trackerfor virtual colonoscopy. Medical Image Computing and Computer-AssistedIntervention (MICCAI), Utrecht, Netherlands, October 2001; Chen et al.,“A Fast Algorithm to Generate Centerline for Virtual Colonscopy”, SPIEConference, Feb. 12-18, 2000. Richard Robb, “Virtual (Computed)Endoscopy: Development and Evaluation Using the Visible Human Datasets”,Oct. 7-8, 1996. www.mayo.edu.; and in U.S. Pat. No. 6,514,082 entitled“System and method for performing a three-dimensional examination withcollapse correction,” issued in the name of Kaufman et al., Feb. 4,2003.

BRIEF SUMMARY OF THE INVENTION

It is herein recognized that prior techniques for centerline calculationsuch as the afore-mentioned techniques that utilize concepts ofmorphological operators, distance transform, minimum cost path,Dijkstra's algorithm, and so forth, are relatively slow.

In accordance with an aspect of the present invention, a method forcenterline calculation exhibits characteristics that are especiallyuseful for colon data.

In accordance with a principle of the invention, it is herein recognizedthat a centerline in a 3D environment can be interpreted as the locationof spheres with maximal diameters constrained by boundary conditions.

In accordance with an aspect of the invention, a method for automaticcenterline extraction for a virtual endoscopy image of an organ having aboundary surface, comprises centering on selected points of an initialpath through the image, derived from an endoscopy dataset, respectivespheres exhibiting respective maximal diameters short of contacting theboundary surface; and forming a centered path consecutively joiningcenters of the spheres.

In accordance with another aspect of the invention, the step of forminga centered path comprises smoothing the centered path to form a modifiedcentered path.

In accordance with another aspect of the invention, the step of forminga centered path comprises centering on selected points of the modifiedcentered path respective spheres exhibiting respective maximal diametersshort of contacting the boundary surface; and forming a further modifiedcentered path consecutively joining centers of the spheres.

In accordance with another aspect of the invention, the step of forminga centered path comprises repetitively performing the steps of claims C1and C2 for deriving a final centered path that has been modified to adesired degree.

In accordance with another aspect of the invention, the step ofcentering the respective spheres comprises:

(a) utilizing spheres modeled by respective polyhedra;(b) centering a modeled first sphere that is relatively small comparedwith space available within the boundary surface;(c) checking for collision between vertices of the modeled first sphereand the boundary surface;(d) in the event of a collision being detected at a point of theboundary, deriving a calculated force to move the modeled sphere awayfrom the point of the boundary surface for ending the collision;(e) in the event of at least one of (A) no collision and (B) a collisionhaving been ended, the modeled first sphere is enlarged until acollision is detected, whereupon steps (d) and (e) are repeated until nofurther enlargement of movement of the modeled first sphere is possiblewithout a collision being detected, whereupon:(f) steps (b) through (e) are repeated for each remaining one of thespheres and a centered path is formed by consecutively joining centersof the spheres.

In accordance with another aspect of the invention, step (d) comprisesthe force causing the modeled sphere to move on a plane perpendicular tothe initial path. In accordance with another aspect of the invention,the step of deriving a calculated force comprises achieving aninteractive speed by utilizing spatial and temporal coherence. Inaccordance with another aspect of the invention, the step of centeringthe respective spheres comprises:

(A) utilizing modeled spheres represented by respective polyhedra;(B) centering a modeled first sphere;(C) checking for collision between vertices of the modeled first sphereand the boundary surface;(D) in the event of a collision being detected at a point of theboundary, deriving a calculated force to move the modeled sphere awayfrom the point of the boundary surface for ending the collision;(E) in the event of at least one of (A) no collision and (B) a collisionhaving been ended, the modeled first sphere is enlarged until acollision is detected, whereupon step (D) IS repeated until no furtherenlargement and no further movement of the modeled first sphere ispossible without a collision being detected, proceeding to step (F); and(F)) repeating steps (b) through (f) are repeated for each remaining oneof the modeled spheres and a centered path is formed by consecutivelyjoining centers of the modeled spheres.

In accordance with another aspect of the invention, step (D) comprisesthe force causing the modeled sphere to move on a plane perpendicular tothe initial path.

In accordance with another aspect of the invention, the step of derivinga calculated force comprises achieving an interactive speed by utilizingspatial and temporal coherence.

In accordance with another aspect of the invention, a method forautomatic centerline extraction for a virtual endoscopy image of anorgan having a boundary surface, comprises: deriving an initial paththrough the image between initial and final voxels of an endoscopydataset; centering on selected points of the initial path respectivespheres exhibiting respective maximal diameters short of contacting theboundary surface; and forming a centered path consecutively joiningcenters of the spheres.

In accordance with another aspect of the invention, a method forautomatic centerline extraction for a virtual endoscopy image of anorgan having a boundary surface, comprises: deriving a colonoscopy voxeldataset by using a colonoscopy protocol; deriving an initial paththrough the image between initial and final voxels of an endoscopydataset; centering on selected points of the initial path respectivespheres exhibiting respective maximal diameters short of contacting theboundary surface; and forming a centered path consecutively joiningcenters of the spheres.

In accordance with another aspect of the invention, a method forautomatic centerline extraction for a virtual endoscopy image of anorgan having a boundary surface, comprises: deriving an endoscopy voxeldataset by using an endoscopy protocol; deriving an initial path throughthe image between initial and final voxels of the dataset, the initialpath exhibiting vertices; centering on consecutive ones of the verticesrespective spheres exhibiting respective maximal diameters short ofcontacting the walls; and forming a centered path consecutively joiningcenters of the spheres.

In accordance with another aspect of the invention, a method forautomatic centerline extraction for a virtual endoscopy image of anorgan having a boundary wall, comprises: deriving an endoscopy voxeldataset by using an endoscopy protocol; starting with an initial voxelin the dataset, labeling voxels neighboring the initial voxel with afirst label number of a series of consecutively increasing labelnumbers; labeling voxels neighboring respective ones of the voxelshaving the first label number with a second label number of the series;repeating the foregoing step by labeling with progressively higher labelnumbers those voxels neighboring voxels numbered in the foregoing step,until an endpoint is reached with voxels having a highest label number;starting at a first voxel with the highest label number, searching for aneighboring second voxel with a smaller label number than the firstvoxel and storing the location thereof; starting at the second voxel,searching for a neighboring third voxel with a smaller label number thanthe second voxel and storing the location thereof; repeating theforegoing step until the initial voxel is reached, thereby establishingan initial path through the image between the initial and the firstvoxel; smoothing the initial path so as to result in an intermediatepath, the intermediate path exhibiting a plurality of vertices;centering on a first vertex a sphere exhibiting a maximal diameter shortof contacting the walls; centering on further vertices of the pluralityrespective spheres exhibiting respective maximal diameters short ofcontacting the boundary wall; and forming a centered path consecutivelyjoining centers of the spheres.

In accordance with another aspect of the invention, the step of forminga centered path consecutively joining centers of the spheres comprises:smoothing the centered path; centering on a selected first point of thecentered path a sphere exhibiting a maximal diameter short of contactingthe walls; centering on selected further points of the centered pathrespective spheres exhibiting respective maximal diameters short ofcontacting the boundary wall; and forming a modified centered pathconsecutively joining centers of the spheres.

In accordance with another aspect of the invention, the first point andthe further points comprise vertices of the centered path.

In accordance with another aspect of the invention, the step of forminga centered path consecutively joining centers of the spheres comprises:smoothing the centered path; centering on a selected first point of thecentered path a sphere exhibiting a maximal diameter short of contactingthe walls; centering on selected further points of the centered pathrespective spheres exhibiting respective maximal diameters short ofcontacting the boundary wall; forming a modified centered pathconsecutively joining centers of the spheres; and selectively repeatingthe above-listed steps until the centered path has been yet furthermodified to a desired degree of smoothing.

In accordance with another aspect of the invention, the first point andthe further points comprise vertices of the centered path.

In accordance with another aspect of the invention, a method forautomatic centerline extraction for a virtual endoscopy image of anorgan having a boundary, comprises:

deriving an endoscopy voxel dataset by using an endoscopy protocol;starting with an initial voxel in the dataset, labeling firstneighboring voxels of the initial voxel with a first label number of aprogressive series; labeling next neighboring voxels of the firstneighboring voxels a second label number of the series; repeating theforegoing step until an endpoint is reached with voxels having a finallabel number of the series; starting at a first voxel with the finallabel number, searching for a neighboring second voxel with an priorlabel number than the first voxel and storing the location thereof;starting at the second voxel, searching for a neighboring third voxelwith an prior label number than the second voxel and storing thelocation thereof; repeating the foregoing step until the initial voxelis reached, thereby establishing an initial path through the imagebetween the initial and the first voxel; smoothing the initial path soas to result in an intermediate path; centering on selected points ofthe intermediate paths respective spheres exhibiting respective maximaldiameters short of contacting the boundary; and forming a centered pathconsecutively joining centers of the spheres.

In accordance with another aspect of the invention, a method forautomatic centerline extraction for a virtual colonoscopy image of acolon having a boundary wall, comprising: deriving a colonoscopy voxeldataset by using a colonoscopy protocol: starting with an initial voxelin the dataset, labeling voxels neighboring the initial voxel with afirst label number of a series of consecutively increasing labelnumbers; labeling voxels neighboring respective ones of the voxelshaving the first label number with a second label number of the series;repeating the foregoing step by labeling with progressively higher labelnumbers those voxels neighboring voxels numbered in the foregoing step,until an endpoint is reached with voxels having a highest label number;starting at a first voxel with the highest label number, searching for aneighboring second voxel with a smaller label number than the firstvoxel and storing the location thereof; starting at the second voxel,searching for a neighboring third voxel with a smaller label number thanthe second voxel and storing storing the location thereof; repeating theforegoing step until the initial voxel is reached, thereby establishingan initial path through the image between the initial and the firstvoxel; smoothing the initial path so as to result in an intermediatepath, the intermediate path exhibiting plurality of vertices; centeringon a first vertex a sphere exhibiting a maximal diameter short ofcontacting the walls; centering on further vertices of the pluralityrespective spheres exhibiting respective maximal diameters short ofcontacting the boundary wall; and forming a centered path consecutivelyjoining centers of the spheres.

In accordance with another aspect of the invention, a method forautomatic centerline extraction for a virtual colonoscopy image of acolon having a boundary surface, comprises: deriving a colonoscopy voxeldataset by using a colonoscopy protocol; deriving an initial paththrough the image between initial and final voxels of the dataset;centering on selected points of the initial path respective spheresexhibiting respective maximal diameters short of contacting the boundarysurface; and forming a centered path consecutively joining centers ofthe spheres.

In accordance with another aspect of the invention, apparatus forautomatic centerline extraction for a virtual colonoscopy image of acolon having a boundary surface, comprises: apparatus for deriving acolonoscopy voxel dataset by using a colonoscopy protocol; apparatus forderiving an initial path through the image between initial and finalvoxels from a colonoscopy dataset; apparatus for centering on selectedpoints of the initial path respective spheres exhibiting respectivemaximal diameters short of contacting the boundary surface; andapparatus for forming a centered path consecutively joining centers ofthe spheres.

In accordance with another aspect of the invention, a method forautomatic centerline extraction for a virtual endoscopy image of anorgan having a boundary surface includes centering on selected points ofan initial path through the image, which is derived from an endoscopydataset, respective spheres exhibiting respective maximal diametersshort of contacting the boundary surface; and forming a centered pathconsecutively joining centers of the spheres.

In accordance with another aspect of the invention, a method forautomatic centerline extraction for data set representing an objecthaving a boundary surface, comprises centering on selected points of aninitial path through the object, derived from the dataset, respectivespheres exhibiting respective maximal diameters short of contacting theboundary surface; and forming a centered path consecutively joiningcenters of the spheres.

In accordance with another aspect of the invention the step of forming acentered path comprises smoothing the centered path to form a modifiedcentered path.

In accordance with another aspect of the invention the step of forming acentered path comprises centering on selected points of the modifiedcentered path respective spheres exhibiting respective maximal diametersshort of contacting the boundary surface; and forming a further modifiedcentered path consecutively joining centers of the spheres.

In accordance with another aspect of the invention the step of forming acentered path comprises repetitively performing the steps of claims 1and 2 for deriving a final centered path that has been modified to adesired degree.

In accordance with another aspect of the invention, the step ofcentering the respective spheres comprises (a) utilizing spheres modeledby respective polyhedra; (b) centering a modeled first sphere that isrelatively small compared with space available within the boundarysurface; (c) checking for collision between vertices of the modeledfirst sphere and the boundary surface; (d) in the event of a collisionbeing detected at a point of the boundary, deriving a calculated forceto move the modeled sphere away from the point of the boundary surfacefor ending the collision; (e) in the event of at least one of (A) nocollision and (B) a collision having been ended, the modeled firstsphere is enlarged until a collision is detected, whereupon steps (d)and (e) are repeated until no further enlargement of movement of themodeled first sphere is possible without a collision being detected,whereupon: (f) steps (b) through (e) are repeated for each remaining oneof the spheres and a centered path is formed by consecutively joiningcenters of the spheres.

In accordance with another aspect of the invention, step (d) comprisesthe force causing the modeled sphere to move on a plane perpendicular tothe initial path.

In accordance with another aspect of the invention the step of derivinga calculated force comprises achieving an interactive speed by utilizingspatial and temporal coherence.

In accordance with another aspect of the invention, the step ofcentering the respective spheres comprises: (A) utilizing modeledspheres represented by respective polyhedra; (B) centering a modeledfirst sphere; (C) checking for collision between vertices of the modeledfirst sphere and the boundary surface; (D) in the event of a collisionbeing detected at a point of the boundary, deriving a calculated forceto move the modeled sphere away from the point of the boundary surfacefor ending the collision; (E) in the event of at least one of (A) nocollision and (B) a collision having been ended, the modeled firstsphere is enlarged until a collision is detected, whereupon step (D) isrepeated until no further enlargement and no further movement of themodeled first sphere is possible without a collision being detected,proceeding to step (F); and (F) repeating steps (B) through (E) for eachremaining one of the modeled spheres and a centered path is formed byconsecutively joining centers of the modeled spheres.

In accordance with another aspect of the invention, step (D) comprisesthe force causing the modeled sphere to move on a plane perpendicular tothe initial path.

In accordance with another aspect of the invention, the step of derivinga calculated force comprises achieving an interactive speed by utilizingspatial and temporal coherence.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the detaileddescription which follows, in conjunction with the drawings in which

FIGS. 1 and 2 show an initial voxel path, an initial smoothing step, andfinal centering in accordance with an embodiment of the invention; and

FIG. 3 shows details of centering steps in accordance with an embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

It will be understood that the method and system of the presentinvention are preferably implemented utilizing a programmable digitalcomputer and that the operations herein described are in reference tosuch an implementation. In the context of imaging, terms such as “air”,“lumen”, etc. are typically intended to refer to the correspondingimaging of these features.

In accordance with an embodiment of the present invention, colonsegmentation comprises performing a start- and endpoint calculation, andperforming an initial path calculation as will be described hereinafter.This is followed by path centering and smoothing.

Generally, the method in accordance with the invention starts with acolon dataset that has been obtained using a colonoscopy protocoltypically including bowel preparation and air insufflation. The datasetis segmented by applying a threshold to air and doing connectedcomponent analysis, whereby connected components that do not belong tothe colon are discarded, either automatically or by manual selection.

It is noted, nevertheless, that the method of the invention isapplicable to other virtual endoscopic examinations and, indeedgenerally to a cavity having boundary surfaces.

Furthermore, the method of the invention is applicable to other datasetswhich are not necessarily prepared for virtual endoscopy, such as, byway of example, blood vessels with contrast. The centerline method hasother possible uses, including registration or mapping two centerlinesonto each other, making measurements, defining orthogonal cross sectionsalong a vessel, segmentation, and visualization.

An embodiment of the present method comprises:

-   -   A. Colon Segmentation;    -   B. Start- and Endpoint calculation;    -   C. Initial Path calculation; and    -   D. Path centering and smoothing.

In step A, Colon Segmentation starts with a colon dataset that has beenobtained using a colonoscopy protocol, e.g. bowel preparation, airinsufflation, and so forth. The dataset is segmented by applying athreshold to air, and doing connected component analysis. Connectedcomponents that do not belong to the colon are discarded, eitherautomatically or by manual selection.

In step B, Start- and Endpoint calculation, a distance labeling isperformed, starting from a first voxel that belongs to the colon. Thisfirst voxel is labeled 0, its neighbors are labeled 1, their neighborsare labeled 2, and so forth. A search is then made for the voxel withthe highest label. This is designated as the start point p0. From p0, anew distance label map is created by repeating step B and obtaininganother voxel with the highest number. This is designated as end pointp1.

In step C, Initial Path Calculation, starting at p1, the distance labelsare used to get a path of connected voxel that ends in p0. This is doneby searching among the neighbors of p1 for a voxel with a smaller label,storing the position, then searching among this voxel's neighbors for avoxel with smaller label, and so forth, until p0 is reached. See FIG. 1a. It is noted that the foregoing initial path calculation is given byway of example and that other suitable steps for this calculation may beemployed instead in an alternative embodiment of the present invention.

In Step D, Path Centering and smoothing, the resulting initial path isgenerally jagged and is smoothed by, for example, applying the knowntechnique of Gaussian smoothing. Any vertex is replaced by the weightedaverage of its n neighbors, where n is a constant selected based on thecharacteristics of the type of smoothing desired, where a larger orsmaller value for n will determine the extent of the area over which anaverage is obtained. The process is repeated over a number ofiterations. Any new vertex position is tested for collision with thecolon wall or boundary surface by verifying whether the new coordinatestill lies within the segmented colon. In the event of a collision, thevertex is left at the last collision-free position. The resulting pathmay be visualized by way of a helpful analogy from the field of staticmechanics where the resulting path for this process resembles the pathresulting from pulling apart both ends of a mass-less flexible stringthat goes through the colon. See FIG. 1 b.

This smooth path is centered using spheres with increasing sizes. SeeFIG. 3. It will be understood that a sphere in the present context isrepresented by a polyhedral structure with a sufficient number of facetsfor an acceptably close representation. Such a polyhedron exhibitsvertices, not to be confused herein with the path vertices. A smallsphere is centered at a vertex along the path.

The vertices on this sphere are checked for collision with the colonwall. If vertices are in collision, a translation force is defined andcalculated, based on the sphere normals. This force is used to move thesphere away from the wall. The sphere is constrained to move on a planeperpendicular to the path. If the sphere is no longer in collision, thesize of the sphere is increased and the collision calculation and shiftare repeated. The process stops when the sphere cannot be shifted and/orincreased any further without creating a collision. Thus, the sphereexhibits a maximal size short of colliding with the walls. The center ofthe sphere is now taken as the new position for the vertex. The processrepeats for the next vertex of the trajectory. See FIG. 1 c. After thecentering, the path undergoes another Gaussian smoothing, with collisioncontrol. This time fewer iterations and a smaller neighborhood are used.

A description of a collision detection technique and calculation oftranslation force is given in a publication by Geiger, B., “Real-TimeCollision Detection and Response for Complex Environments,” ComputerGraphics International 2000; Jun. 19-23, 2000; Geneva, Switzerland. Thisarticle, whereof the disclosure is herein incorporated by reference tothe extent not incompatible with the present invention, presents amethod for collision detection that is well suited to complexenvironments, such as those obtained from medical imaging and forobjects that are in permanent contact. The method is based on apoint-intetrahedral-mesh query. Spatial and temporal coherence are usedto achieve interactive speed. In addition to collision detection, thesystem calculates a force and torque that can be used for collisionresponse.

However, the collision detection and force calculation in the presentinvention is preferably done directly on the voxel, rather than onpolyhedral reconstructions, although it generally follows the approachoutlined in the aforementioned paper by Geiger.

In summary, FIGS. 1 a and 2 a show an initial voxel path, FIGS. 1 b and2 b show an initial smoothing step, and FIGS. 1 c and 2 c show finalcentering. FIG. 3 at a shows a centering step in which a sphere is setat the vertex location. In FIG. 3 at b the sphere size is increaseduntil it collides with the wall. From the collision, a translation forceis calculated. In FIG. 3 at c, the translation is applied until thesphere is no longer in collision. The sphere size is increased once moreand it now collides with the wall. At d a translation is calculated.After translation has occurred, the sphere reaches a position where itcannot grow any further. This is the final vertex position at e in FIG.3.

The primary example used is that of a virtual colonoscopy; however, themethod of the invention is applicable to other virtual endoscopicexaminations and, indeed generally to a cavity having boundary surfaces.

It is particularly emphasized that the method of the invention isapplicable to other datasets which are not necessarily prepared forvirtual endoscopy, such as, by way of example, blood vessels withcontrast, as has been stated above. The centerline method has otherpossible uses, including registration or mapping two centerlines ontoeach other, making measurements, defining orthogonal cross sectionsalong a vessel, segmentation, and visualization.

The invention has been described by way of exemplary embodiments. Itwill be apparent to one of ordinary skill in the art to which itpertains that various changes and substitutions may be made withoutdeparting from the spirit of the invention. For example, as will beappreciated, the consecutive numbering of voxels is conveniently made inascending numerical order and it is apparent that a descending sequenceor any other labeling ordinal sequence of labeling can be used. Foranother example, variations in the method of derivation of the initialpath may be made in an equivalent manner. Thus, other suitable ways ofderiving the initial path may be substituted for the steps disclosedabove by way of example for the steps of Colon Segmentation, Start- andEndpoint calculation, and/or Initial Path calculation. Given a suitableinitial path, the step of Path centering and smoothing can then becarried out.

These and similar variations and substitutions are contemplated in thepresent invention which is defined by the claims following.

1-24. (canceled)
 25. A method for automatic centerline extraction for avirtual endoscopy image of an organ having a boundary surface,comprising: deriving an endoscopy voxel dataset by using an endoscopyprotocol; deriving an initial path through said image between initialand final voxels of said dataset, said initial path exhibiting vertices;centering on consecutive ones of said vertices respective spheresexhibiting respective maximal diameters short of contacting said walls;and forming a centered path consecutively joining centers of saidspheres.
 26. A method as recited in claim 25, wherein: said step offorming a centered path comprises smoothing said centered path to form amodified centered path.
 27. A method as recited in claim 26, whereinsaid step of forming a centered path comprises: centering on selectedpoints of said modified centered path respective spheres exhibitingrespective maximal diameters short of contacting said boundary surface;and forming a further modified centered path consecutively joiningcenters of said spheres.
 28. A method as recited in claim 26, whereinsaid step of forming a centered path comprises repetitively performingthe steps of claims C1 and C2 for deriving a final centered path thathas been modified to a desired degree. 29-58. (canceled)